This is a division of copending application Ser. No. 388,101, filed on July 31, 1989.
1. Field of Invention
The present invention relates generally to a connecting rod having a hollow section and a method for casting the connecting rod. More particularly, the present invention relates to a process for casting the hollow connecting rod precisely to a near net shape, the hollow connecting rod having a thin cylindrical wall to produce desirable stiffness to weight characteristics. The invention also relates to an apparatus for performing the method.
2. Description of the Related Art
As is well known, many engines, such as an internal combustion engine, require for their operation at least four basic parts: (1) a cylinder; (2) a piston; (3) a connecting rod; and (4) a crank shaft. Reciprocating linear movement of the piston within the cylinder, produced by the burning of fuel, results in the generation of power from the engine. When the piston is driven downward by the pressure of expanding gases in the cylinder, one end of the connecting rod moves downward with the piston in a straight line. The lower end of the connecting rod moves down and in a generally circular motion at the same time. Together, the crank shaft and connecting rod transmute linear to rotary motion.
During this process, the connecting rod is subjected to tensile, compressive, and bending forces. The high bending stresses are superimposed on the forces associated with tension and compression. Bending stresses tend to cause buckling, even where the connecting rod may be strong enough to withstand other forces exerted thereupon.
To meet challenges posed by fuel economy standards and competitive pressures, improvements in connecting rods for internal combustion engines have included the adoption of light weight designs which are capable of withstanding the stresses developed in internal combustion engines while operating at high rates of power output. When producing high horsepower at elevated rotational speeds, common failures include the breaking of connecting rods due to the enormous stresses developed under such operational conditions.
Often, the most destructive loads imposed on connecting rods are those imposed at top dead center by inertia during the exhaust stroke of the engine. In this configuration, the inertia loads are at a maximum level, while opposing gas forces are at a minimum. Such inertia forces are transmitted to the connecting rod with the result that failure occurs with consequent damage to or destruction of the engine. Solutions include decreasing the weight of the reciprocating parts, such as the connecting rod and the piston, or increasing the strength of the connecting rod. Such approaches have long been recognized and have been disclosed in, for example, U.S. Pat. No. 3,482,468.
To meet design needs such as those described above, several types of connecting rods have been provided heretofore, including rods of an I- or H-beam cross-section, and solid cylindrical or tubular shank members. Each type of rod, however, tends to be somewhat heavy and cannot be operated for any substantial period at high speeds without failure. Modes of imminent or actual failure include deformation of the portion of the rod connected to the piston and deformation or destruction of the portion of the rod connected to the crank shaft, together with bending or breakage of the rod as a result of forces acting thereupon. While the H-beam type of rod often has greater rigidity than either the solid, tubular, or I-beam types of rods, even the H-section rod sometimes has poor characteristics at high rotational speeds, which causes failure of the rods.
Tubular connecting rods that have previously been known exhibit considerable stiffness or rigidity up to a point where flexing during operation exceeds the elastic limit of the rod. Buckling results beyond the elastic limit, which causes damage to the engine.
Currently produced connecting rods are typically made from either a forged steel, cast iron, or powder metallurgical process. Wall thicknesses attainable by forging may only be as low as about 3.0 mm. By powder metallurgical techniques, only about 2.8 mm in wall thickness can be realized. Thus, the designer's need for lightness remains unsatisfied by conventional approaches. Also, the section connecting the small and large ends of the connecting rod typically has rough surface features which result from the manufacturing process. Furthermore, significant costs are usually associated with finishing conventional connecting rods and in drilling bolt holes which accommodate bolts which fasten the connecting rod to the crank shaft.
To produce hollow cast articles such as connecting rods, core molds have conventionally been used in known casting processes. Such cores are typically prepared by shaping a core material from an aggregate mixture, such as alumina or zirconia, and a binder, such as ethyl silicate, and then baking the core. However, it is difficult to prepare cores with precise dimensions since conventional cores tend to expand during baking or shrink during drying or cooling, thereby losing their dimensional stability. Other problems of conventional core approaches are that core quality is often irregular, thereby jeopardizing production efficiency and often leading to high production costs because of the need to resort to expensive remedial measures.
Under conventional processes, precise casting is elusive because the core tends to move within the die before and during injection of a molding material, thereby thwarting the attainment of a connecting rod conforming to a near net shape. After melting and removal of molding material from the shell mold, the problems of secure retention by the shell mold of the core are exacerbated by hydrodynamic forces associated with the swirling flow of molten metal.